There has been a problem in a two component developing system with regard to its performance degradation. One of the causes of the problem is performance degradation of a carrier. Such performance degradation of the carrier includes adhesion of toner to the carrier (so-called “spent phenomenon”), peeling of coating applied to a surface of a carrier particle, and the like. Conventionally, there have been proposed various systems for suppressing such performance degradation of carrier.
For example, Japanese Examined Patent Publication, No. 111598/1987 (Tokukohei 7-111598, publication date: Jun. 10, 1987: Patent document 1) and Japanese Examined Patent Publication, No. 21591/1984 (Tokukohei 2-21591, publication date: Jun. 9, 1984: Patent document 2) describe developing devices using a so-called “trickle” system. In the developing devices disclosed in these publications, a small amount of carrier is supplied as toner is replenished, while an excess of the carrier (or the mixture of carrier and toner) in a developing device is discharged. This allows carrier in the developing device to have performance changing within a certain range, thereby achieving stability in the performance of the carrier over time.
In the trickle system, it is important to discharge developer by an amount corresponding to an amount of replenished developer (carrier), so as to maintain a certain amount of the developer in a developing device. This is because fluctuations in amount of the developer in the developing device adversely affect image quality.
However, in the developing devices disclosed in Patent documents 1 and 2, the developer is discharged by a natural discharging system, which causes the developer to overflow (overflow system). Specifically, the developer is added by an amount of replenished developer, and the developer is discharged by an amount exceeding the level of a discharging opening. This allows fluctuations in amount of the developer in the developing device to be within a predetermined range.
In such a simple overflow system, discharge of developer is subject to an inclination of a main body of an image forming apparatus, so that discharge of developer in a developing device becomes excessive, or inhibited contrary.
With reference to FIGS. 16(a) through 16(c), the above operations are described below in more detail. In a developing device, a discharging opening 100 is provided on one of the edges (right edge in the figures) of a developer tank 101 in its longitudinal direction. As shown in FIG. 16(a), under a normal (horizontal) setting condition of the developing device 100, when the developer tank 101 contains developer with a predetermined amount of Z(g), a top face of the developer in the developer tank (indicated by a dash-dotted line) is at a level of a bottom end of the discharging opening 100. Thus, the amount of developer in the developer tank is stabilized to be the predetermined amount of Z(g).
However, as shown in FIG. 16(b), when the developing device is inclined so that the end portion having the discharging opening 100 becomes lower than the other end, the top surface (indicated by a dash-dotted line) of the developer with the amount of Z(g) exceeds the top face of the discharging opening 100. This causes the developer to be discharged until the top face of the developer comes to the bottom end of the discharging opening 100. As a result, the amount of the developer in the developer tank 101 is stabilized to be an amount of Z−X (g), which is less than the predetermined amount of Z(g) by the amount of X(g).
On the contrary, as shown in FIG. 16(c), when the developing device is inclined so that the end portion having the discharging opening 100 becomes higher than the other end, the top surface (indicated by a dash-dotted line) of the developer with the amount of Z(g) does not reach the top face of the discharging opening 100. This causes the developer to be replenished until the top face of the developer comes to the bottom end of the discharging opening 100. As a result, the amount of the developer in the developer tank 101 is stabilized to be an amount of Z+Y (g), which is larger than the predetermined amount of Z(g) by the amount of Y(g).
As a method for suppressing changes in amount of the developer due to the inclinations of a developing device, for example, an excess of the developer is discharged by (i) providing a shutter member on a wall surface of a developer tank, (ii) detecting a predetermined amount of printing or an amount of the developer in a developing device, and (iii) opening or closing the shutter member based on the detection signals. Further, Japanese Patent Publication, No. 3034736 (publication date: May 12, 1995: Patent document 3) discloses a developing device in which a discharging opening is provided on the middle portion of the developing device in its longitudinal direction, so that inclinations of the developing device have less effect on the amount of the developer.
However, in the trickle system, since an excess of the developer is a trace, there is a difficulty in accurately discharging such a minute amount of the developer using a shutter member. This causes difficulties in maintaining a predetermined amount of the developer with high accuracy.
Further, the method as disclosed in Patent document 3, in which the developer discharging opening is disposed on the middle portion of the developing device in its longitudinal direction, is not effective for a developing device that performs circulating and agitating operation using a screw and that is used in color coping machines or the like requiring high performance capabilities for the agitating operation. This is because, even in the case where the discharging opening is disposed on the middle portion of the developing device in its longitudinal direction, the circulating and agitating operation of a screw is significantly affected by inclinations of the surface of the developer. Further, in the developing device of Patent document 3, the method is not effective with regard to inclinations in a direction orthogonal to the longitudinal direction.
Further, Publication of Japanese Patent, No. 3603492/1998 (publication date: Feb. 20, 1998: Patent document 4) discloses a developing device having a plurality of discharging openings. In the developing device in this publication, the discharging openings are provided at different positions including a position where the flow of developer retains and a position where the flow of the developer is stable. This makes it difficult for the developing device to achieve stability in amount of the developer when the developing device is inclined.